Huh. Didn't realize that after the Verizon acquisition, it lost all ownership ties with Yahoo. So future readers don't have to click through:
Yahoo Japan was always a separate company, a joint venture between Yahoo and SoftBank. When Verizon acquired Yahoo, the bits that Verizon didn't want - including Yahoo's share of Yahoo Japan - were spun off into a holding company, Altaba. And so now SoftBank and Altaba between them own most of Yahoo Japan.
Yahoo Japan was always a separate company, a joint venture between Yahoo and SoftBank. When Verizon acquired Yahoo, the bits that Verizon didn't want - including Yahoo's share of Yahoo Japan - were spun off into a holding company, Altaba. And so now SoftBank and Altaba between them own most of Yahoo Japan.